The effect of CPAP on weight and fluid homeostasis will be assessed. Obstructive sleep apnea treatment-naive subjects will randomized to CPAP and control groups. Weight change, and changes in overnight urinary volume, intra and extracelullar volume, plasmatic B-type natriuretic peptide (BNP) and antidiuretic hormone (ADH) and urinary osmolality will be compared between groups.

There is a bidirectional relationship between Obstructive Sleep Apnea (OSA) and obesity. Although obesity can lead to OSA, patients with OSA are more likely to gain weight. The mechanisms that lead to weight gain during the treatment of OSA are not known. Some evidence supports the hypothesis that fluid accumulation could be a possible explanation for weight gain after treatment for OSA. The hypothesis is that one night of CPAP treatment promotes increased body weight and reduced diuresis in patients with OSA. Participants will be submitted to 2 consecutive nights of polysomnography (PSG). A baseline PSG will be performed during the first study visit. During the second study night, participants will be randomized into two groups, CPAP and control (repeat baseline PSG). Overnight urinary volume will be determined. Intra and extracellular water volume will be assessed using electrical bioimpedance before bedtime and just after the subject wakes up. Plasmatic BNP and ADH and urinary osmolality will also be determined during each morning following PSG.

Inclusion Criteria:

• Patients with a diagnosis of Obstructive Apnea Sleep
• Over 18 years of age
• No previous treatment with Continuous Positive Airway Pressure

Exclusion Criteria:

• Congestive heart failure
• Renal insufficiency
• Hepatic failure
• Urinary incontinency
• Diuretic therapy